1. Technical Field
The present invention relates generally to the field of power distribution. More particularly, the present invention relates to a method and system for eliminating certain harmonics in a distributed power system.
2. Background Information
Distributed power systems are employed in a large number of power generation applications. In particular, the use of small and medium-size distributed power systems in a variety of applications has risen in recent years.
Conventional distributed power systems comprise either an alternating current (AC) power source or direct current (DC) power source, or a combined AC and DC power source. Such systems provide power to an end load, e.g., an end user, through an output inverter. End loads comprise either linear loads, or non-linear loads, and/or a combination of both linear and nonlinear loads. The use of nonlinear end loads in such systems has increased over the past several years. For example, the use of adjustable speed drives and uninterruptible power supplies (UPS) has increased a great deal over the past several years. Such nonlinear end loads conventionally comprise a rectifier front end which converts AC power to DC power.
The operation of nonlinear end loads often involves the injection of harmonic current into the power system. The presence of such harmonics in the power system reduces the power quality level. Many organizations that set related standards, such as the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC), publish standards that restrict harmonics content to a defined level. According to IEEE standard, distributed power systems must be designed to deliver xe2x80x9cclean powerxe2x80x9d to a linear load with very low Total Harmonics Distortion (THD), e.g., less than five percent (5%). In particular, each individual harmonics component should be less than three percent (3%). Conventional distributed power systems are designed to deliver power to a linear end user load with very low THD that is in compliance with the IEEE standard.
Referring to FIG. 1, a block diagram of a conventional distributed power system is illustrated. The conventional distributed power system 10 includes an inverter 12 for receiving DC supply current 14 from a power source 16 and producing output current 18 for driving linear or non-linear load 20. Power source 16 may include AC power generator 22 and/or DC power source 24. For example, alternating current 26 from AC power generator 22 is rectified into direct current 28 by rectifier 20, and DC bus 32 delivers the direct current to inverter 12 as supply current 14. Additionally or alternately, direct current 34 from DC power source 24 is converted by DC/DC converter 36 from one voltage level 34 to another voltage level 38, and DC bus 32 delivers the direct current to inverter 12 as supply current 14. Inverter 12 converts the DC supply current into an AC output current 18. Output current 18 comprises both a fundamental frequency current and a harmonic frequency current. In order to reduce the fundamental frequency error current, controller 40 is operable for receiving the output current 18, comparing the output current 18 with a reference current (not shown), determining the fundamental frequency error current, and providing an adjustment current thereby adjusting the output current 18 accordingly.
Typically, however, when such a conventional distributed power system is connected to a nonlinear end load (e.g., a nonlinear end load at a customer""site), the THD in the power system is unacceptably high, sometimes failing the harmonic standards set forth by standards-providing organizations. The high THD in the power system is, in large part, the result of harmonics components injected into the power system by the nonlinear end load.
The harmonics injected into the distributed power system creates a problem for the supplier of the distributed power system. The magnitude of the THD injected by the nonlinear end load of various customers can vary greatly. The unpredictable harmonics injected by a customer""s load makes it difficult for the supplier to provide xe2x80x9cclean powerxe2x80x9d to end loads. Conventional systems do not provide an effective, simple, and practical solution to such a problem. What is needed is an effective, simple, and practical solution for the supplier of distributed power systems to reduce or eliminate harmonic distortion injected by nonlinear loads to distributed power systems.
It is a feature of embodiments of the present invention to provide an effective, simple, and practical solution for the supplier of distributed power systems to reduce or eliminate the effect of harmonics injected by nonlinear loads on distributed power systems. It is a further feature of embodiments of the present invention to provide a method and system for improving power quality levels in distributed power systems.
It is a further feature of embodiments of the present invention to provide a method and system that reduces or eliminates unwanted harmonics by adding an additional negative feedback control loop in the distributed power system. It is a further feature of embodiments of the present invention to provide a method and system for reducing or eliminating such harmonics that may be used with existing distributed power systems.
In one embodiment, a distributed power system comprises an inverter having a reference current, an output current and a summation component. The reference current comprises a reference fundamental frequency current and at least one reference harmonic current. The output current comprises an actual fundamental frequency current and at least one actual harmonic current. The summation component is operable to combine a first signal representative of the output of the fundamental current regulator and a second signal representative of the output of the harmonic current regulator to generate a third signal for driving the inverter to produce the output current. This DPS further comprises a control system in communication with the inverter, the control system operable for receiving a feedback signal representative of the output current and generating the harmonic error current representative of a difference between the reference harmonic current and the actual harmonic current.
In another embodiment, a distributed power system comprises an inverter having a reference current, an output current and a summation component. The reference current comprises a reference fundamental frequency current and at least one reference harmonic current. The output current comprises an actual fundamental frequency current and at least one actual harmonic current. The summation component is operable to combine a first signal, representative of the output of the fundamental current regulator, and a second signal, representative of the output of a harmonic current regulator, to generate a third signal for driving the inverter to produce the output current. The DPS further comprises a control system in communication with the inverter. The control system has a fundamental frequency current controller and a harmonics current controller. The fundamental frequency current controller may be operable for receiving a feedback signal representative of the output current and generating the fundamental frequency error current. The harmonics current controller may be operable for receiving the feedback signal and generating the harmonic error current. The fundamental frequency error current is representative of a difference between the reference fundamental frequency current and the actual fundamental frequency current. The harmonic error current is representative of a difference between the reference harmonic current and the actual harmonic current. The DPS further comprises a filtering component within the harmonics controller for removing predetermined harmonics components from the feedback signal and a transformer system for orienting the feedback signal into the same reference frame as the reference current.
In yet another embodiment, a method is provided for eliminating or substantially reducing certain harmonics in a distributed power system. The method comprises the steps of receiving an output current from a inverter; filtering predetermined harmonics components from the output current to obtain harmonics feedback current; comparing the harmonics feedback current with a reference harmonics current to obtain a harmonics error current; and adding the output of the harmonics current controller to the fundamental current controller output.
In a further embodiment, a method of eliminating or substantially reducing certain harmonics in a distributed power system is provided. The method comprises receiving harmonic current from an output of a power inverter, wherein the power inverter is connected to a nonlinear end load; transforming the output current from stationary frame to synchronous frame at predetermined harmonic frequency; generating a harmonic current regulating signal by summing the negative of the output harmonic current and an associated reference harmonic component; transforming the harmonic current regulating signal from synchronous reference frame back to stationary reference frame of the power inverter; and summing the harmonic current regulating signal and the fundamental current regulating signal in the power inverter.